Gneiss (Schistose)

Physical Properties

Hardness: 6-7 (Mohs); Color: Gray, white, silvery-green, and tan; Luster: Dull to pearly (due to mica content); Crystal Structure: Coarse-grained with visible banding/foliation; Cleavage: Generally poor, though it may split along mica-rich layers.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Under extreme heat and pressure, minerals recrystallize and segregate into the distinct linear bands seen on the surface. These specimens are often hundreds of millions to billions of years old.

Uses & Applications

Used primarily in construction as crushed stone for road bases or as decorative dimension stone for countertops, flooring, and landscaping. Occasionally used in stone masonry.

Geological Facts

Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling' or 'to glisten.'

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—distinct layers of light and dark minerals. This specific specimen shows signs of being water-worn (rounded) and contains significant mica or chlorite giving it a slight greenish-silver sheen. Common in mountainous regions/cratons.

Identified on: 4/26/2026

Mode: Standard